Methods of applying droplets of an electrically conductive fluid are known in the art. For example, EP2373451 describes a method to eject droplets of a fluid, such as a molten metal or a molten semiconductor on a recording medium. Droplets are ejected by generating a Lorentz force in the fluid close to an orifice of the droplet ejection device, and as a result of this Lorentz force, droplets are ejected through the orifice onto the recording medium.
This technique may be applied to jet material at a high temperature and is therefore suitable for jetting electrically conductive materials having a high melting point. Examples of such materials are molten metals, such as silver (melting point (mp)=972° C.), copper (mp=1085° C.), gold (mp=1064° C.) and titanium (mp=1668° C.). Alternatively, an example of such materials are molten semiconductors, such as germanium (mp=938° C.). Jetting of droplets of silver or copper onto a silicon recording medium may be useful in the production of solar cells. For producing solar cells, lines of a material comprising silver and/or copper may be applied onto a silicon surface. These lines comprising silver and/or copper may function as an electrode in the solar cell. In order to obtain a highly efficient solar cell, it is important to have good electric conductivity, both through the electrodes and between the electrodes and the silicon substrate. It was found that there is room for improvement regarding the electrical conductivity within solar cells produced by printing lines of silver and/or copper onto a silicon substrate using the method as disclosed in EP2373451.
It is an object of the invention to provide an improved method for applying an image of an electrically conductive material onto a recording medium.
It is a further object of the invention to provide a device for carrying out the improved method.
The first object of the invention is achieved in a method for applying an image of an electrically conductive material onto a recording medium, the method comprising the steps of:                a) selecting an electrically conductive material and selecting a recording medium comprising a first material, wherein the electrically conductive material and the first material are capable of forming an eutectic alloy;        b) heating at least a part of the recording medium; and        c) ejecting a droplet of the electrically conductive material onto the part of the recording medium,wherein the at least a part of the recording medium is heated to a temperature above a melting point of the first material.        
In the method according to the present invention, an image may be build on a recording medium by applying droplets of an electrically conductive material onto the recording medium. Using the method according to the present invention, an image may be formed that is electrically conductive. The conductivity of the image may depend e.g. on the material applied onto the recording medium and on the image itself. For example, if the image formed is continuous, for example a line, or an area, then the image of the electrically conductive material may be electrically conductive itself. Forming conductive patterns on a recording medium may be useful, e.g. in the production of solar cells or in the production of other electronics. By printing a line of a conductive material, such as silver, copper or an alloy thereof, an electrode may be formed on the recording medium. In the method according to the present invention, in step a), an electrically conductive material and a recording medium comprising a first material are selected, wherein the electrically conductive material and the first material are capable of forming an eutectic alloy. Examples of electrically conductive material-first material combinations that are capable of forming an eutectic alloy are e.g. Al—Ge, Au—Ge, Al—Si, Au—Si, Cu—Si and Ag—Si.
In the method according to the present invention, at least a part of the recording medium is heated in step b). In step c), droplets of the electrically conductive material are ejected onto the part of the recording medium. Step b) may be performed before step c). Alternatively, step b) and step c) are performed simultaneously. In both cases, the recording medium may be at least locally heated to a temperature above a melting point of the first material.
In the method according to the present invention, in step b), at least a part of the recording medium is heated. The recording medium may be heated to a temperature above the melting point of the first material in step b). The recording medium may be heated by suitable heating means. The recording medium may be heated locally. Alternatively, the entire recording medium may be heated. In case the recording medium comprises a (semi-)conductive material, the recording medium may be heated e.g. by applying an electrical current to the recording medium. Alternatively, the recording medium may be heated by placing the recording medium (and optionally also the jetting device used) in a controlled environment having an elevated temperature. Optionally, the recording medium may be heated using a plurality of heating means. Thus, the first material may be molten when a droplet of the electrically conductive material is jetted on the recording medium. If the recording medium consists essentially of the first material, then the recording medium should preferably be locally molten. For example, a thin layer of the surface of the recording medium may be molten, whereas the bulk remains solid. In case the recording medium comprises a plurality of compounds, the first material may melt upon heating the recording medium, whereas other components comprised in the recording medium may not melt.
Local melting of the first material may be achieved e.g. by locally irradiating the recording medium with a laser. Optionally, also the heat of a droplet applied onto the recording medium may be used to increase the temperature of the first material to a temperature above the melting point of the first material. If appropriate, the recording medium may be irradiated during short time-interval, to ensure that only a small part of the recording medium melts, leaving intact the framework formed by the recording medium. By applying a droplet of molten fluid on the recording medium, the first material and the electrically conductive material may mix to some extent. Mixing may take place e.g. because of the impact of the droplet on the partially molten recording medium. In addition, also diffusion may cause mixing of the materials, especially at higher temperatures.
When the materials are mixed, a eutectic alloy may be formed upon cooling, said alloy comprising both the first material and the electrically conductive material. Depending on the amount of electrically conductive material applied and on the mixing of the materials, also (pure) electrically conductive material may solidify. A three layer structure may be formed, wherein the recording medium comprises a layer of the first material and is further provided with a layer of the eutectic alloy and a layer of the electrically conductive material. Optionally, solid particles of the eutectic alloy and of the electrically conductive material may be mixed to some extent.
The formation of the eutectic alloy may improve conductivity between the image of the electrically conductive material and the recording medium. For example, it was found that in case of an Ag—Si eutectic alloy, the contact resistance may be lower in a system comprising the Ag—Si eutectic alloy compared to a system not comprising the eutectic alloy.
It was surprisingly found that the use of a of electrically conductive material-first material combination that is capable of forming an eutectic alloy, in combination with heating of the recording medium may improve the efficiency of solar cells produced by said method. Heating of the at least part of the recording medium may slow down the cooling of the electrically conductive material after is has been deposited on the recording medium and may thereby slow down solidification of the material. Without wanting to be bound to any theory, it is believed that this may have at least two beneficial effects. In the first place, a droplet of the electrically conductive material may stay liquid until an adjacent droplet is applied onto the medium. When two fluid droplets are positioned in close proximity to one another, then the droplets may merge to some extent. In case a line of droplets is printed to form an electrode and all droplets merge to some extent, then good connectivity between the droplets applied onto the recording medium—and thus, within the electrode—is provided, which may result in good electrical conductivity. This may improve the efficiency of a solar cell comprising such electrode. In the second place, the droplet of the electrically conductive material is applied onto a recording medium having an increased temperature, which may slow down cooling and solidification of the droplet of fluid. If the droplet stays fluid for an increased period of time, the fluid may flow to some extent over the surface of the recording medium, improving the contact of the electrically conductive material and the recording medium. If the recording medium comprises a (semi-) conductive material, the electrical conductivity within an assembly of the recording medium and the image of the electrically conductive material applied thereon may be further improved by the improved contact between the electrically conductive material and the recording medium. In addition, adhesion of the image onto the recording medium may improve if the contact surface between the droplet of electrically conductive material and the recording medium increases by improved flow of the droplet.
In an embodiment, step b) and c) are performed simultaneously. The recording medium may be pre-heated to an elevated temperature. The elevated temperature may, but does not necessarily have to be a temperature above the melting temperature of the first material. Pre-heating of the recording medium may be done e.g. by using a laser or by keeping the recording material in an environment at elevated temperature. The droplets of the electrically conductive material that are jetted onto the recording medium may be at a high temperature, such as a temperature above the melting temperature of the first material. When the droplets of the electrically conductive material are at a temperature above the melting temperature of the first material, then temperature of the first material may be further increased. This temperature increase may occur locally, i.e. at a position of droplet impact. Hence, only at a position of droplet impact, where local melting of the first material is desired, the temperature of the first material may be increased to a temperature above the melting point of the first material. The amount of first material that melts upon impact may depend on a plurality of parameters, e.g. on the temperature of the droplet, the size of the droplet, the heat capacity of the droplet and the first material and on the temperature of the first material before droplet impact. Hence, by using the method according to this embodiment, melting of the first material can be suitably controlled.
In an embodiment, a droplet of the electrically conductive fluid is ejected by applying a Lorentz force to the fluid. This force causes a motion in the conductive fluid. This motion may cause a part of the fluid to move from the fluid chamber through the orifice, thereby generating a droplet of the fluid. The Lorentz force is related to the electric current and the magnetic field vector; {right arrow over (F)}={right arrow over (I)}×{right arrow over (B)}. The Lorentz force resulting from the electric current and the magnetic field is generated in a direction perpendicular to both the electrical current and the magnetic field. By suitably selecting the direction and the magnitude of the electric current, as well as the direction and the magnitude of the magnetic field, the direction and the magnitude of the resulting Lorentz force may be selected. In the system according to the present invention, in normal operation, the magnetic field is provided and an electrical current is provided in the conductive fluid, such that a suitable force for ejecting a droplet is generated.
The jetting device in accordance with the present invention comprises a fluid chamber and has an orifice extending from the fluid chamber to an outer surface of the fluid chamber element. In operation, the fluid chamber comprises an electrically conductive fluid. When applying an actuation pulse, a Lorentz force is generated within the fluid, causing the fluid to move through the orifice in a direction away from the fluid chamber. The actuation pulse may be applied by applying a pulsed magnetic field and a continuous electrical current, or a pulsed electrical current in a continuous magnetic field, or a combination thereof. Alternatively, a constant Lorentz force may be generated within the fluid by applying a constant electrical current to the electrically conductive fluid in a constant magnetic field. However, application of a constant Lorentz force to the electrically conductive fluid may result in the ejection of a stream of the electrically conductive fluid, instead of in the ejection of droplets.
Other techniques may be applied as well to eject droplets of the electrically conductive fluid, such as piezo actuation or thermal inkjet. However, actuation based on Lorentz force actuation may be better suited to eject droplets of a fluid at high temperatures. For example, if droplets of fluid silver are to be jetted, the temperature of the fluid is at least 972° C., which is the melting point of silver.
In an embodiment, in step b) of the method according to the present invention, the recording medium is heated using a laser. A laser provides a narrow beam of radiation. If the beam is pointed towards the recording medium, the recording medium may be locally irradiated by the laser beam. Upon irradiation of the recording medium by the laser beam, energy is provided at least locally to the recording medium, which may result in heating the recording medium at least locally. Different types of lasers are known in the art. The person skilled in the art will be able to suitably select a laser for use in the method according to the present invention, based e.g. on the type of recording medium applied.
In an embodiment, the electrically conductive material is a metal or a semiconductor. Metals and semiconductors are materials that in general have a good electric conductivity. For example, ejecting droplets of a molten metal may be used to print electronic circuits. By applying a printing technique to apply droplets of metal onto a recording medium, electronic circuits may be printed in a versatile way; i.e. the shape of the circuit may be easily adapted for the respective electronic circuits.
In an embodiment, the electrically conductive material has a melting point of at least 300° C. If the electrically conductive fluid has a high melting temperature, such as a melting temperature of 300° C. or higher, the fluid may cool down rapidly after ejection through the nozzle of the jetting device, because of possible large temperature differences between environment and droplet. In that case, it may be important to suitably control the temperature of the recording medium to suitably control the cooling of the droplet after ejecting and after application on the recording medium. In an embodiment, the electrically conductive material has a melting point of at least 600° C. In a further embodiment, the electrically conductive material has a melting point of at least 900° C.
In a further embodiment, the electrically conductive material is selected from at least one of the group consisting of silver, gold and copper. Silver and copper may be usefully applied in electronic devices, for example as electrodes. In addition alloys comprising at least one of copper and silver may be used. By applying an image of silver, copper or an alloy thereof onto a recording medium, electrically conductive parts, such as electrodes, may be efficiently applied onto the recording medium.
In an embodiment, the recording medium is a silicon substrate. In case the recording medium is a silicon substrate, the method according to the present invention may be used to produce a solar cell. By providing the silicon substrate with an image of an electrically conductive material, the silicon substrate may be provided with electrodes and a functioning solar cell may be produced. The silicon substrate may be e.g. a polycrystalline silicon substrate, a monocrystalline silicon substrate, a nanocrystalline silicon substrate or an amorphous silicon substrate. Optionally, the silicon substrate may be provided with a coating. For example, the coating may be applied on a side of the silicon substrate that may not be provided with an image of the electrically conductive material.
Alternatively, metallization of silicon substrates may be usefully applied in the manufacturing of integrated circuits (IC's) for use in electronic equipment, e.g. computers or mobile phones.
In an embodiment, the method further comprises the step of:
α) modifying the recording medium,
wherein step α) is carried out before step a). The recording medium may be modified to further optimize the surface properties of the recording medium. For example, an oxide layer, for example a silicon oxide layer, may be removed from the surface. This may be done e.g. via an etching process, or via reduction of the oxide containing layer using H2. Alternatively, the surface of the recording medium may be coated with a suitable coating. For example, a thin layer of metal may be deposited on the recording medium, for example via screen printing or via Chemical Vapor Deposition (CVD). Alternatively, a thin layer of glass frit may be deposited on the recording medium.
In an embodiment, the electrically conductive material has a higher melting point than the first material. Preferably, the melting point of the electrically conductive material may be at least 50° C. higher than the melting point of the first material. For example, gold may be selected as the electrically conductive material, whereas germanium is selected as the first material. Gold has a melting point of 1064° C., whereas germanium has a melting point of 938° C. The phase diagram of a gold/germanium system shows a eutectic point, corresponding to a eutectic temperature of 356° C.
The droplet of the electrically conductive fluid expelled through a nozzle may have a temperature higher than the melting point of the first material. For these systems, the first material may be melted by applying a droplet of the electrically conductive fluid onto the recording medium. After the droplet has been applied onto the recording medium and the first material is—at least—partially molten, mixing of the two components may take place, e.g. because of diffusion or because of motion of the fluid. Upon cooling of the systems, the eutectic composition may be formed.
In a further aspect of the invention, a device for ejecting droplets of an electrically conductive fluid onto a recording medium is provided, the device comprising:                a fluid chamber body defining a fluid chamber and having an orifice extending from the fluid chamber to an outer surface of the fluid chamber element, the fluid chamber being at least partly arranged in a magnetic field;        magnetic field generating means for generating a magnetic field;        an actuation means comprising at least two electrically conductive electrodes, each electrode being arranged such that one end of each electrode is in electrical contact with the fluid in the fluid chamber;        heating means for heating the electrically conductive material, the device further comprising:        heating means for heating at least a part of the recording medium.        
The device in accordance with the present invention comprises a fluid chamber body defining a fluid chamber and having an orifice extending from the fluid chamber to an outer surface of the fluid chamber element. The fluid chamber is adapted to comprise the electrically conductive fluid. The fluid chamber body comprises an orifice, the orifice extending from the fluid chamber to an outer surface of the fluid chamber body. Hence, a droplet of fluid may be ejected from the fluid chamber body via the orifice. The device according to the present invention also comprises actuation means. The actuation means may be means for actuating a droplet of the electrically conductive fluid by applying a Lorentz force.
The fluid chamber body is preferably made of a material, which is cost effectively machinable. The material for the fluid chamber body may be selected from, but is not limited to graphite, boron-nitride or silicon. When the fluid chamber body is made from silicon, the fluid chamber body may be made using etching techniques, for example. Alternatively, the fluid chamber body may be formed starting from a molten material which is poured into a mould and is cooled down such that the molten material solidifies. Afterwards, the mould may be removed yielding the fluid chamber body. In another embodiment, at least a part of the fluid chamber body may be made from a solid piece, using a mechanical technique, such as milling, drilling, sawing or the like, or a chemical technique, such as etching or the like. The fluid chamber body may be made out of one piece or may be assembled starting from a plurality of pieces using a suitable assembly technique, such as welding, gluing, nailing or the like.
Depending on the type of electrically conductive fluid, such as a molten metal or a molten semiconductor to be jetted, the fluid chamber body may need to be heat resistant. Preferably the body is resistant to temperatures up to 3000 K, which enables to handle a large range of electrically conductive materials. Also, it may be preferred that the material is resistant against corrosion by the electrically conductive fluids. Furthermore, the material of the fluid chamber, in particular at the orifice, is wettable by the fluid. If the fluid does not wet on the material, the fluid will contract itself and bead and consequently will not flow through the orifice when no (external) force is exerted on the fluid. By ensuring that the fluid chamber body is wettable by the fluid, the fluid will flow into the orifice without an additional force. For example, the fluid chamber may be coated with a tungsten carbide, such as WC, W2C and W3C.
In an embodiment, the material of the fluid chamber body is not electrically conductive. This may be advantageous for preventing an electrical actuation current flowing into the fluid chamber body, since such a flow into the body material might decrease a generated actuation force. In a particular embodiment, the fluid chamber body comprises boron-nitride (BN).
Moreover, the electrically conductive material is a material that in the fluid state has an electric conductivity that is sufficient to generate a Lorentz force that is strong enough to eject a droplet of the fluid through the orifice, upon applying a certain current, in a given magnetic field and geometry of the fluid chamber body. Therefore, whether a fluid is an electrically conductive fluid in accordance with the present invention should be determined taking into account the conditions, for example the current and the magnetic field applied and on the geometry of the fluid chamber body.
The device in accordance with the present invention may further comprise heating means for heating the electrically conductive material. The electrically conductive fluid may be solid at ambient conditions. Therefore, the electrically conductive material may have to be molten. This may be done using suitable heating means. In addition, the temperature of the electrically conductive fluid may have to be further increased once it is molten. The temperature of a droplet of the electrically conductive material may influence the temperature of the recording medium. By applying droplets onto the recording medium that have a temperature higher than the temperature of the recording medium, the temperature of the recording medium may be increased. Optionally, the heat of the droplet may locally heat the recording medium to a temperature above the melting temperature of the recording medium. A Non-limiting example of suitable heating means for heating the electrically conductive fluid is an inductive coil provided around the fluid chamber body.
The device in accordance with the present invention may further comprise heating means for heating at least a part of the recording medium. The heating means may be e.g. a laser, or—in case the recording medium is electrically conductive—electrical current proving means for providing a current through the recording medium, thereby heating said medium. Preferably, the heating means are adapted such that the heating means are suited to locally heat the recording medium. The device may further comprise suitable positioning means for positioning the heating means with respect to the recording medium and with respect to the fluid chamber body.
In an embodiment, the heating means for heating the recording medium comprises a laser. A laser may be a suitable heating means, because it may locally irradiate the recording medium, thereby heating said recording medium. Since the beam of a laser may be essentially non-divergent, small parts of the recording medium may be irradiated selectively, whereas other parts of the recording medium may not be irradiated. Therefore, selected parts of the recording medium, such as parts of the recording medium to be provided with the image of the electrically conductive material, may be heated selectively. Different types of lasers are available varying a.o in intensity of the beam and wavelength of the radiation emitted. The person skilled in the art will be able to suitable select a laser, depending e.g. on the type of recording medium used and the desired temperature to be reached.
In an embodiment, the device further comprises a control unit configured to control the heating means for heating at least a part of the recording medium. The control unit may suitably control the heating means. For example, the control unit may control the amount of heat supplied to the recording medium. Optionally, the control unit may also control the parts of the recording medium which are heated, and which parts are not. For example, different parts of the recording medium may be heated to the respective desired temperatures, which may be different for the respective parts of the recording medium, depending on the print strategy requirements. Furthermore, the control unit may also control the other parts of the device for ejecting droplets of an electrically conductive fluid. For example, the control unit may control the amount of droplets that are ejected and/or may control the temperature of the droplet. By suitably controlling the heating means, it may be prevented that the recording medium is overheated, such that the recording medium melts. In addition, it may be ensured that sufficient heat is provided by the heating means, such that the temperature of the recording medium is within a predefined temperature range. The amount of heat that is to be provided by the heating means may depend e.g. on the material of the recording medium, the type of electrically conductive material, the amount and temperature of the electrically conductive material applied onto the recording medium, and the desired temperature of the recording medium.
In an embodiment, the control unit may comprise a feedback loop. The temperature of the recording medium may be measured during the jetting process using suitable temperature measuring means. When the temperature exceeds a predefined limit, the control unit may prevent the heating means from providing heat to the recording medium. When the temperature falls below another predefined limit, the control unit may start the heating means to apply heat. In that way, the temperature of the recording medium may be kept within a predefined temperature range. In a further embodiment, the control unit may detect the temperature of the recording medium and further compare this to a second temperature limit, the second temperature limit being higher than the predefined limit. For example, when the temperature exceeds the second temperature limit, there may be a risk of overheating (and possibly melting) of the recording medium. When the temperature exceeds this second temperature limit, the control unit may not only pause heating of the recording medium, but may also pause jetting of droplets onto the medium.
In an alternative embodiment, the control unit may comprise a feed-forward loop. This is particularly useful if the droplets of the electrically conductive fluid have a relatively large influence on the temperature of the recording medium. For example, if the droplets have a temperature higher than the temperature of the recording medium, the temperature of the recording medium may rise when a droplet of the electrically conductive material is applied to it. Alternatively, if the droplets have a relatively low temperature, the temperature of the recording medium may drop when droplets of the electrically conductive material are applied onto the recording medium. Thus, the temperature of the recording medium may depend on the amount and on the rate at which droplets of the electrically conductive material are applied to it. The data concerning the amount of droplets and the rate at which they are applied may be stored in a print job. Thus, based on the print job, the control unit may adapt the heating means to suitably heat the recording medium such that the temperature of the recording medium stays within the desired temperature range.